Investments (Loan-to-value ratios) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Commercial Mortgage Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	$ 192,764
2019	361,798
2018	106,707
2017	283,961
2016	286,509
Prior	360,056
Revolving Loans	0
Recording investment gross of allowance for credit losses	1,591,795	$ 1,414,139
Commercial Mortgage Loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
2020	192,764
2019	338,722
2018	90,462
2017	283,961
2016	282,478
Prior	347,475
Revolving Loans	0
Recording investment gross of allowance for credit losses	1,535,862	1,361,868
Commercial Mortgage Loans | 1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	23,076
2018	16,245
2017	0
2016	4,031
Prior	9,804
Revolving Loans	0
Recording investment gross of allowance for credit losses	53,156	52,271
Commercial Mortgage Loans | Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	2,777
Revolving Loans	0
Recording investment gross of allowance for credit losses	2,777	0
Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	36,765
2019	13,769
2018	1,261
2017	8,265
2016	1,180
Prior	32,549
Revolving Loans	0
Recording investment gross of allowance for credit losses	93,789	60,046
Agricultural Property Loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
2020	36,765
2019	13,769
2018	1,261
2017	4,277
2016	1,180
Prior	28,957
Revolving Loans	0
Recording investment gross of allowance for credit losses	86,209	56,437
Agricultural Property Loans | 1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Recording investment gross of allowance for credit losses	0	0
Agricultural Property Loans | Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	3,988
2016	0
Prior	3,592
Revolving Loans	0
Recording investment gross of allowance for credit losses	7,580	3,609
0%-59.99% | Commercial Mortgage Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	116,872
2018	26,733
2017	83,965
2016	195,585
Prior	255,804
Revolving Loans	0
Recording investment gross of allowance for credit losses	678,959	665,858
0%-59.99% | Commercial Mortgage Loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		659,217
0%-59.99% | Commercial Mortgage Loans | 1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		6,641
0%-59.99% | Commercial Mortgage Loans | Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
0%-59.99% | Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	34,465
2019	13,769
2018	1,261
2017	8,265
2016	1,180
Prior	32,549
Revolving Loans	0
Recording investment gross of allowance for credit losses	91,489	60,046
0%-59.99% | Agricultural Property Loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		56,437
0%-59.99% | Agricultural Property Loans | 1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
0%-59.99% | Agricultural Property Loans | Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		3,609
60%-69.99% | Commercial Mortgage Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	140,998
2019	172,412
2018	32,176
2017	139,016
2016	61,536
Prior	96,476
Revolving Loans	0
Recording investment gross of allowance for credit losses	642,614	513,571
60%-69.99% | Commercial Mortgage Loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		499,493
60%-69.99% | Commercial Mortgage Loans | 1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		14,078
60%-69.99% | Commercial Mortgage Loans | Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
60%-69.99% | Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	2,300
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Recording investment gross of allowance for credit losses	2,300	0
60%-69.99% | Agricultural Property Loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
60%-69.99% | Agricultural Property Loans | 1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
60%-69.99% | Agricultural Property Loans | Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
70%-79.99% | Commercial Mortgage Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	51,766
2019	72,514
2018	47,798
2017	60,980
2016	29,388
Prior	7,776
Revolving Loans	0
Recording investment gross of allowance for credit losses	270,222	233,713
70%-79.99% | Commercial Mortgage Loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		203,158
70%-79.99% | Commercial Mortgage Loans | 1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		30,555
70%-79.99% | Commercial Mortgage Loans | Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
70%-79.99% | Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Recording investment gross of allowance for credit losses	0	0
70%-79.99% | Agricultural Property Loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
70%-79.99% | Agricultural Property Loans | 1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
70%-79.99% | Agricultural Property Loans | Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
80% or greater | Commercial Mortgage Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Recording investment gross of allowance for credit losses	0	997
80% or greater | Commercial Mortgage Loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
80% or greater | Commercial Mortgage Loans | 1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		997
80% or greater | Commercial Mortgage Loans | Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
80% or greater | Agricultural Property Loans
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Recording investment gross of allowance for credit losses	$ 0	0
80% or greater | Agricultural Property Loans | ≥ 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
80% or greater | Agricultural Property Loans | 1.0X to 1.2X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		0
80% or greater | Agricultural Property Loans | Less than 1.0X
Financing Receivable, Credit Quality Indicator [Line Items]
Recording investment gross of allowance for credit losses		$ 0